this does not make sense
 

I have a completely stock 1951 ford victoria with stock v8 with stick shift. I've had a slight noise and vibration from the drive train, so I jacked up both rear wheels to spin them and see if i could hear anything. Here's where it gets crazy, both wheels spin freely with no drag or noise but the drive shaft does not turn. it doesn't matter weather it's in gear or in neutral and yet if I put the wheels back on the ground the car drives normally with just this slight noise and vibration. I'm being told this is impossible but it's true.

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Yikes!

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With both wheels off the ground you need to check that the wheel on the opposite side to the one you are turning by hand is chocked so that it will not turn. If there is any friction from the output of the transmission or the input of the differential this can make the opposite wheel rotate in the reverse direction to the wheel you are rotating by hand, if you follow my logic??? :)

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With my limited knowledge if you spin the rear wheels the driveshaft has to turn, if it's in gear you won't be able to turn the wheels because you would be trying to turn the engine, if its in neutral the wheels and driveshaft will turn together. Evidently right now in my case when the engine is running and the trans is in gear the driveshaft will put power to the wheels but with the car off the ground turning the wheels does not engage the driveshaft>

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Re-read my reply above :)

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Just curious. With it in gear and engine running if both wheels are spinning, then take an old bath towel,,,,,,,,or your wife's new one,,,,,,,,,and lay on your back and reach around both sides of the left tire and starting dragging it to a stop with the towel. Just keep pressure on it, What happens now?

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What you have posted is not correct. If you turn one wheel, the other wheel will turn in the opposite direction and the driveshaft will not be turning. This is how it is designed to work, if it didn't you would have difficulty turning a corner. A rear axle has a ring and pinion set of gears, but it also has a set of what are sometimes called spider gears. These gears allow the wheel turning you are referring to.

See if this helps any: https://www.bing.com/videos/search?q...7&&FORM=VDRVRV

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meric 42
just went out and chocked one rear wheel, when i spin just one wheel the driveshaft turns, when i was spinning both wheels together the drive shaft wasn't turning.

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Raise only one of the wheels off the floor and try again.

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thanks merc42 and JSeery- I just didn't understand the sequence and learn something new every day. Appreciate your knowledge and help. I assume I have no problem.

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Yep, your axle and drive shaft are working just like they should. :)

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except that he has a slight vibration. grab the u joints on the drive shaft and see that they are snug

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1937 GM training film on spider gear operation. This is a good video explaining how spider gears work.

https://www.youtube.com/watch?v=yYAw79386WI

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The vibration could be worn U joints. What does the noise sound like? Have you checked the fluid in the transmission and rear end?

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FF, awesome video, thanks for posting.

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Vibration could also be drive shaft out of balance.

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Bias ply tires with flatspots from sitting?

Spin the tires while watching them and make sure they are not starting to separate?

Tire balance?

You can try rotating the tires.

U-joints, like everybody said?

When you replace u-joints its a good practice to keep the front slip yoke, driveshaft and even the pinion yoke in their original positions

Later year vehicles with two-piece driveshafts need to have their u-joints all lined up (in phase)

Center support bearings on two-piece driveshafts.

Bent driveshaft?

Transmission tailshaft bushing alowing the driveshaft yoke to move excessively?

Axle bearings pitted?

Engine or trans mounts making metal to metal contact? It just takes one little spot to not be insulated by the rubber mounts

See if the vibration is there while on the gas, coasting in gear, coasting in neutral?

Do you feel the vibration in the seat of your pants or in the steering wheel?

Do you feel the vibration parked, while revving the engine in neutral. Maybe something is out of balance like a clutch?

At work we would put the vehicle up in the air on the hoist and have someone drive it at 50 mph. While you stand under there next to the whirling driveshaft, wearing a stethoscope and listening to the pinion bearings, axle bearings..... I've even stuck another mechanic in the trunk to listen and feel for a vibration and gone for a ride. We would also stick the vehicle on the smog machine's dyno so there was a load on the drivetrain to try and simulate a vibration.

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Nice video.

How does the differential not turn the driveshaft though? In conclusion. I guess if you only turn one wheel (or not lock one wheel) the spiders just turn in the differential :).



It's nice to get a straight forward explanation. Not a just because, that's how it is.

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Tinker, The spinning wheel is directly geared to the spider, which is in turn geared 1/1 to the opposite wheel. The spider offers considerably less resistance to the turning, whereas the ring gear offers more resistance to turn the pinion/driveshaft, due to the gearing ratio. The result is the opposite wheel is turned reverse to the spinning wheel.

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Flathead Fever
At work we would put the vehicle up in the air on the hoist and have someone drive it at 50 mph. While you stand under there next to the whirling driveshaft, wearing a stethoscope and listening to the pinion bearings, axle bearings..... I've even stuck another mechanic in the trunk to listen and feel for a vibration and gone for a ride. We would also stick the vehicle on the smog machine's dyno so there was a load on the drivetrain to try and simulate a vibration.

We did the same years ago. That is why i mentioned stopping one tire with a towel. Stop one and then do the other. We use to try to isolate axle bearings this way. Did it work? Usually. Some axle bearings were very loud. But some times not.